Non-erasable, or write-once, data storage media such as digital optical disks, paper tape, PROMS, etc., have a plurality of bit positions, each of which can be irreversibly changed from an original state to a new state but one time. Thus, typically, the initial state of the bit position is designated a "zero" and the "zero" can be overwritten with a "one" when data is written into the media. Once the data has been written in a section of the media, which may be all of the media, that section is considered to be "used" by those experienced in the field, and is not considered to be reusable thereafter for recording new data.
Some non-erasable media, notably digital optical disks, can store vast amounts of data. A single twelve-inch disk can be used to store over 10.sup.11 bits of data, the equivalent of forty reels of magnetic tape, and access can be provided to any of it in about one-tenth of a second. This is an order of magnitude improvement in the cost/performance of memory technology and has had dramatic effects. And, while the cost of a high capacity, single twelve-inch disk may be only $100, that high capacity and low cost are achieved only at the further philosophical cost of making the writing process irreversible.
Similar limits exist, and are probably more familiar, in connection with the non-erasable media, such as punched paper tape, punched cards, and programmable read only memories (PROMS). However, the tremendous capacity and low cost per bit of digital optical disks provides a strong motivation for more closely examining their one drawback, the non-erasable nature of the memory storage process.
Therefore a primary object of this invention is an apparatus and method for reusing, that is writing more than once in, non-erasable memories. Other objects of the invention are an apparatus and method for reliably and simply increasing the effective capacity of non-erasable memories, and for providing a simple, easily implemented apparatus and method for rewriting in an otherwise non-erasable memory.